1. Field of the Invention
This invention relates to handoff control for point to multipoint connections in mobile ATM networks, and for the first time provides for handoff control for a mobile participating in a point to multipoint connection.
This invention relates to a system for handoff control in a point to multipoint mobile ATM network. The invention is embodied in a system, a method, and a program product for handoff control in a point to multipoint mobile ATM network
2. Related Art
Asynchronous transfer mode (ATM) networks provide for point to point (PTP) and also for point to multipoint (PMP) connections. In PTP connections, one station communicates with only one other station.
In PMP connections, one station broadcasts to a plurality of other stations. The station so broadcasting may be referred to as a broadcasting station or a root station. The stations so receiving the broadcast of the root station may be referred to as receiving stations or as leaf stations. PMP connections are useful when it is desired to send a broadcast to several stations, for example, in an educational lecture setting. Using a PMP connection over an ATM network, a root station at a university could broadcast a lecture to students participating at leaf stations.
Stations in an ATM network connect to the network at switching nodes of the network. ATM switching nodes may be interconnected by links. In a PMP connection, the node to which the root station connects may be referred to as the root node of the PMP connection. Similarly, the nodes to which leaf stations connect may be referred to as leaf nodes. Obviously, a leaf node may provide service to more than one leaf station. Communication that moves in the direction away from the root station toward the leaf stations may be referred to as downstream communication; communication that moves in the direction toward the root station may be referred to as upstream communication.
FIG. 1 shows a plurality of ATM switching nodes. Some of the nodes are interconnected by links. The nodes are represented by circles, and the links are represented by straight lines between the nodes.
Today, ATM networks may include support for mobile terminals. In mobile ATM networks, a mobile terminal (or, simply, a mobile; also referred to as a MT) communicates with the ATM network via a base station (BS). The BS, for the purposes of this discussion, may conceptually be considered as part of a switching node. It is not necessary for all switching nodes to be BS""s. A BS may be considered to be a special kind of switching node having communication facilities for directly communicating with MT""s. Mobile ATM networks provide for handoff for PTP connections when an MT in a PTP connection moves from an area served by one BS into an area served by another BS. That is, a MT may have its PTP ATM connection handed off between different BS""s. The BS that the MT leaves may be referred to as the old base station, or OldBS. The BS that the MT goes to may be referred to as the new base station, or NewBS.
Some confusion is possible in using the term xe2x80x9ccellxe2x80x9d when discussing mobile ATM networks. This confusion arises because mobile networks have historically used xe2x80x9ccellxe2x80x9d to indicate the service area of a BS. In other words, a MT moving from one cell to another will have its call handed off between an OldBS and a NewBS. In ATM networks, however, xe2x80x9ccellxe2x80x9d has been used to refer to the ATM cell which serves as the basic unit for protocol processing and switching. To avoid confusion, herein the term xe2x80x9ccellxe2x80x9d refers to the ATM cell unless otherwise indicated, and the service area of a BS will be referred to as a service area.
An ATM network may operate according to a Private Network to Network Interface (PNNI) hierarchy. The PNNI hierarchy provides for scalability of networks and is highly advantageous. The PNNI hierarchy provides that peer entities may be grouped together. A conceptual overview of the PNNI hierarchy will now be given.
FIG. 2 shows one way in which the nodes of FIG. 1 might be grouped at a high level. FIG. 2 does not show the links between the nodes for the sake of clarity.
More particularly, the nodes above the dashed line may be thought of as belonging to an overall group referred to as group B. The nodes below the dashed line may be thought of as belonging to a group A. Group A and group B are defined at the same high level, and may be referred to as peers of each other. That is, group A is a peer of group B.
FIG. 3 shows a lower level grouping of nodes. Again, the links between the nodes have been omitted for clarity. In particular, the nodes of peer group B have been grouped into groups B.1 and B.2; the nodes of peer group A have been grouped into groups A.1, A.2, A.3, and A.4. It will be appreciated that these lower level groups are peers of each other. That is, groups B.1 and B.2 are peers of each other and may also be referred to as peer groups. Groups A.1, A.2, A.3, and A.4 are peers of each other.
At its lowest level, a network may be understood to include a plurality of nodes, each which has a switching station or the like. Since these nodes are all at the same level, they are peers.
By convention, a switching node may be named based on the names of the groups of which it is a part. Thus, a switching node named A.2.1 may be in highest level group A, in the next level group A.2, and may be switching node number 1 within group A.2. Hence, the identifier or name xe2x80x9cA.2.1xe2x80x9d. This naming convention may be referred to as a hierarchical naming convention.
FIG. 4 shows how the switching nodes in the exemplary network might be named under the foregoing convention.
The PNNI hierarchy thus provides for peer groups of an arbitrary number of levels of abstraction. The scalable PNNI hierarchy helps hide from upper levels the impact of changing the network at lower levels, and also helps hide from other peer groups any changes made inside one peer group.
To support PMP connections, a PNNI ATM network requires that a PMP connection must have a consistent tree topology at every level. More particularly, the root of the tree in a PMP connection is the root station. The leaves of the tree in a PMP connection are the leaf stations. The leaves must connect to the root via branches which do not overlap or cross. The prohibition of branch overlap/crossing allows a PMP connection to exist in harmony with the scalability of the PNNI network over all of the different levels of abstraction.
The foregoing tree topology requirement imposed by the PNNI hierarchy does not substantially affect handoff for PTP ATM connections during handoff between BS""s. PTP handoff may be accomplished in a straightforward manner. The foregoing tree topology requirement does, however, pose serious implications for PMP ATM connections during handoff. In particular, unless the proper handoff control is provided, it is possible that, when a MT participating as a leaf station in a PMP connection moves from the service area of the OldBS to the NewBS, the handing off of the connection to the NewBS might cause two branches impermissibly to cross or overlap.
This situation will be explained with respect to an example and FIGS. 5-12. FIG. 5 shows the exemplary network with the switching node addresses labeled, and the links between switching nodes as straight lines. In FIG. 5, there is a root station RT which is connected to switching node B.2.4. A first leaf station, L1, is connected to switching node A.2.3. A second leaf station, L2, is connected to switching node A.4.4.
FIG. 6 shows a PMP connection established through the ATM network by which L1 and L2 receive communications from RT. In FIG. 6, peer group A.1 in its entirety, several other switching nodes, and several links have been omitted for clarity. The PMP connection is shown as a heavy, dark line. Links not part of the PMP connection are shown as faint lines. The PMP connection includes switching nodes B.2.4, B.2.3, and B.2.2 of peer group B.2; switching nodes B.1.1 and B.1.2 of peer group B.1; switching nodes A.3.2, A.3.1, and A.3.4 of peer group A.3; switching node A.2.3 of peer group A.2; and switching nodes A.4.6 and A.4.4 of peer group A.4.
FIG. 7 shows only a part of the exemplary ATM network with the PMP connection now including a third leaf which is a mobile terminal MT. The MT is in communication with switching node A.4.2, and the PMP connection includes switching nodes A.4.3 and A.4.2 in addition to the switching nodes already mentioned. Since the connection between MT and A.4.2 is a mobile communication link, it is shown as a heavy dashed line.
FIG. 8 shows the MT in a highly schematic fashion. In particular, a transmit and receive unit 10 may have an antenna ANT through which radio communications are received and sent. Connected to transmit and receive unit 10 may be a processing unit 20 which enables the MT to participate in wireless radio communications. FIG. 9 shows an exemplary switching node which is a base station in a mobile ATM network in highly schematic fashion. The terms xe2x80x9cbase stationxe2x80x9d and xe2x80x9cswitching nodexe2x80x9d may be considered to be the same for many purposes of this discussion. In particular, a base station 60 includes a base station transmit and receive unit 30 having at least one antenna ANT. The base station 60 may also have a base station processing unit 40 which controls unit 30 to receive and send radio communications through antenna ANT. The base station 60 may also include a switching unit 50 which interfaces with the links of an ATM network.
Switching unit 50 may include a processor and an associated memory. The memory may include instructions adapted to enable the processor to cause the switching unit to participate in predefined ways in the ATM network. A switching node that is not a base station might not have units 30, 40, or ANT.
FIG. 10 shows a base station BS 60 and its service area 70. Generally, a MT in the service area 70 of a BS may communicate with the switching unit 50 of the ATM network via the BS 60. FIG. 11 shows how the service areas 70 of different BS""s 60 may be provided in close proximity one to another so as to provide substantially continuous communications capability.
Suppose MT, which presently is communicating via switching node A.4.2, is traveling closer to the service area of A.2.2. The signal from A.4.2 is decreasing in strength and the signal from A.2.2 is increasing. When the relative strength of these two signals reaches a certain threshold, communications should be handed off from A.4.2 to A.2.2.
FIG. 12 shows how the PMP connection would appear if such a handoff is performed in a straightforward manner. In FIG. 12, several more nodes and links not presently relevant have been omitted for improved clarity. The PMP connection is shown as being extended from A.4.2 to A.2.2 so that MT can continue to participate in the PMP connection, and MT is shown as communicating with the ATM network via A.2.2.
Such a handoff is impermissible, however, because the extension of the PMP connection between A.4.2 and A.2.2 violates the necessary tree topology. In particular, this impermissible connection would provide two branches from peer group A.3 that terminated in peer group A.2. To put it another way, those two branches may be said to xe2x80x9ccrossxe2x80x9dor to xe2x80x9coverlapxe2x80x9d at peer group A.2. The required tree topology of a PMP connection in an ATM network thus would be violated.
The fact that connection handoff can violate the tree topology presents a serious problem with respect to the support of PMP connections in mobile ATM networks.
Therefore, mobility is not presently supported for PMP connections in ATM networks. Moreover, there are multiple types of PMP connections, and this further complicates handoff control. The multiple types of PMP connections will now be briefly discussed.
According to ATM Forum specifications (see background documents 12, 13), there are three types of PMP connections. The three types of PMP connections are the Root Initiated PMP connection, the Root LIJ connection, and the Network LIJ connection. Each will now be discussed in turn, but it is important to keep in mind that these three types of PMP connections are defined without regard to mobile ATM networks. In other words, these three types of PMP connections are specified for ATM networks regardless of whether the particular ATM network provides wireless access.
Root Initiated PMP connections are created by the root and only the root can initiate signaling procedure by sending an ADD PARTY message toward a new leaf.
A Root LIJ PMP connection is characterized in that it is a root prompted, Leaf Initiated Join (LIJ). Root LIJ connections allow a leaf to request to join the PMP connection by sending a LEAF SETUP REQUEST message to the root. Upon receiving the message, the root starts the signaling procedure to add this leaf by sending an ADD PARTY message toward the new leaf.
A Network LIJ PMP connection is characterized in that it is a leaf prompted Leaf Initiated join (LIJ). Network LIJ connections allow a leaf to request to join the PMP connection by sending a LEAF SETUP REQUEST message toward the root. This request message might not reach the root. The network determines whether there exists a node that qualifies as a proxy root for the leaf. If there is a proxy root for the leaf, then the proxy root (and not the xe2x80x9crealxe2x80x9d root) starts the signaling procedure to add the leaf by sending the ADD PARTY message toward the new leaf. In Network LIJ PMP connections, the nodes upstream of the proxy root might not notice the leaf""s join because the join is handled by the proxy root. Network LIJ is the only type of ATM PMP connection in which a proxy root is used.
ATM networks have heretofore been studied and defined in various aspects. The following documents are listed for the convenience of the reader, as they contain useful background information on these various aspects, and are all incorporated by reference in their entirety for this useful background information:
Concerning the xe2x80x98Mobile ATMxe2x80x99 concept:
1. D. Raychaudhuri, R. Yuan, A. Iwata, and H. Suzuki. Rationale and framework for wireless ATM specification. ATM Forum/95-1646/PLEN, 1995.
Concerning the near term set of mobile services in a mobile ATM network:
2. Acharya, J. Li, A. Bakre, and D. Raychaudhuri. Design and prototyping of location management and handoff protocols for wireless ATM networks. In Proceedings of ICUPC 1997, San Diego, September 1997.
Concerning a long term migration to broadband end-to-end wireless ATM service:
3. D. Raychaudhuri and N. D. Wilson. ATM-based transport architecture for multiservices wireless personal communication networks. IEEE Journal on Selected Areas in Communications, 12(8):1401-1414, December 1994.
Concerning research and development into mobility support for ATM networks:
4. Acampora and M. Naghshineh. An architecture and methodology for mobile-executed handoff in cellular ATM networks. IEEE Journal on Selected Areas in Communications, 12(8):1365-1375, December 1994.
5. K. Toh. Crossover switch discovery for wireless ATM LANs. ACM/Baltzer Mobile Networks and Nomadic Applications, 1(2), December 1996.
6. R. Yuan, S. K. Biswas, L. J. French, J. Li, and .D. Raychaudhuri. A signaling and control architecture for mobility support in wireless ATM networks. ACM/Baltzer Mobile Networks and Applications, 1(3), December 1996.
7. M. Veeraraghavan, M. Karol, and K. Eng. Mobility and connection management in a wireless ATM LAN. IEEE Journal on Selected Areas in Communications, 15(1):50-68, January 1997.
8. H. Mitts, H. Hansen, J. Immonen, and S. Veikkolainen. Lossless handover for wireless ATM. ACM/Baltzer Mobile Networks and Applications, 1(3), December 1996.
Concerning efforts toward standardization:
9. Rajagopalan H. Mitts, K. Rauhala and G. Bautz. Proposed handover signaling architecture for release 1.0 WATM baseline. ATM Forum/97-0845, September 1997.
10. A. Acharya, J. Li, and D. Raychaudhuri. Primitives for location management and handoff control in mobile ATM networks. ATM Forum/96-1121, August 1996.
Concerning a framework for mobility support in an ATM network:
11. Acharya, J. Li, and D. Raychaudhuri. Mobility management in wireless ATM networks. IEEE Communication Magazine, 35(11):100-109, November 1997.
Concerning specifications for user-network and network-network interfaces:
12. The ATM Forum. ATM User-Network Interface (UNI) Signalling Specification, Version 4.0. ATM Forum/af-sig-0061, July 1996.
13. The ATM Forum. Private Network-Network Interface Specification (PNNI) Version 2.0. ATM Forum/BTD-PNNI 2.00, September 1997.
Concerning selection of a COS in PTP handoff:
14. J. Li, A. Acharya, and D. Raychaudhuri. A signaling mechanism for hand-off control in mobile ATM networks. In Proceedings of the 12th International Conference of Information Networking, Tokyo, Japan, January 1998.
This invention is realized in a method of handoff for all three types of PMP connections in a mobile PNNI ATM network in such a manner as to avoid branch crossing/overlapping. The invention also is realized in a computer system and a computer program product for implementing the foregoing method.
The handoff control method provides for the discovery of a proper cross over switch which is an entry border node that covers both the old base station and the new base station. The inventive control method includes new protocol messages, a new type of cell for providing handoff without any data loss, and a new PNNI route view to support setting up of new paths for any of the three types of PMP connections.